1. Field of the Invention
The present invention relates to a method for bonding together two discs having information recording layers and, more specifically, to an optimal method for manufacturing a DVD ("digital video disc" or "digital versatile disc") employing an bonding method.
2. Description of the Related Art
In the conventional method for bonding two plate-like objects together employing a UV curable resin as an bonding agent, a UV curable composition is first coated uniformly to the bonding surfaces of the plates using a known technology such as spin coating, screen printing or the like. The two plates are then stacked so that the bonding surfaces meet. UV light is then irradiated in a continuous emission to cure the resin.
A high-pressure mercury lamp, metal halide lamp, mercury-xenon lamp or other such conventionally used device may be employed as the light source in the UV irradiation. However, since the lamp continuously emits light in this method, heat is readily generated, which can cause warping of the disc or impair its mechanical characteristics.
Further, in these types of continuous emission lamps, it generally requires several minutes or more for the light emission to stabilize once the lamp is turned on. Accordingly, since these lamps cannot be turned on and off readily, the lamp must be left on when a product is being produced continuously. For example, if a production cycle (i.e., the time required for a single bonding process) requires 5 seconds, with UV irradiation carried out for 2 seconds, then energy is wasted during the remaining 3 seconds in which the lamp is on but not needed.
Moreover, when bonding two plate-like objects together using a UV curable resin as an bonding agent, fewer problems occur if at least one of the two plate-like objects is UV transmissive.
However, when manufacturing a product such as a DVD, which is formed by bonding together two plate-shaped disc substrates, a thin metallic layer or other layer which does not readily transmit UV may be present under the bonding layer. In such cases, the intensity of the UV light is significantly weakened by these types of layers, so that efficient curing of the bonding agent may not occur.
Further, if the curing of the bonding agent is to be promoted, it then becomes necessary to provide a large capacity lamp so that the cost of the finished product unavoidably increases. Moreover, if a large capacity lamp is employed, then the heat given off by the lamp presents a problem in that it may give rise to warping or other deformations in the discs. In order to avoid this, it becomes necessary to provide separate cooling equipment around the lamp, making the device larger and more complicated.
Conversely, UV curing may be carried out using a small capacity lamp, but irradiation must then be performed for 20 to 30 seconds or more, making actual application of this approach difficult.